Formatted Search working Partially on SDK Field

I have a text field on the Service call screen called

"A"

I need the field to be updated with a value taken from the Item Master table, U_A field.

I have created my query as below,

SELECT I.U_A FROM OITM I

WHERE I.ItemCode = $[$33.0.0]

and set a formatted search on the field A in the Service Call screen to refresh regularly when the Item No field changes.

The Formatted search does NOT fire when the item number changes.

It fires only when I do Shft + F2

HOWEVER if I try the formatted search on a system field on the same screen such as the Description field, then the Item Number Change fires the formatted search.

Does anyone know of this resriction, or a workaround please?

appreciate your help.

Thanks,

Indika